1. Field of the Invention
This invention relates to a method of rapid filtration of a chemical bath having particles therein of greatly varying sizes.
2. Brief Description of the Prior Art
Chemical baths, particularly such baths used in the fabrication of semiconductor devices, become contaminated with particles that may enter the bath from the semiconductor wafer introduced into the bath, from the chemical processing of the wafer, from the ambient and from countless other sources. Such contaminants can vary in size from very small to very large. It is necessary to remove these contaminants from the bath with increased requirement for removal of the very small particles as the geometry of the semiconductor devices is diminished.
Contaminant removal involves recirculation of the bath fluids through filters prior to reintroduction of the bath fluids back into the bath. The normal convention for recirculating and filtering a solution is to input the filtered solution into the tank bottom at the center of the tank and allow the solution being used in the tank to overflow over the tank top and into a wier with the overflow then being extracted from the bottom of the wier. In order to insure removal of the smaller particles, a small pore filter has been used which was capable of removing the smaller particles of interest. A problem with the use of such small pore filters is that the flow of fluid in the recirculation path is limited, thereby preventing rapid removal of all particles and particularly the larger particles. When larger pore sized filters have been used in place of the small pore filters, in situ particle monitors (ISPMs) have revealed a lower total particle count than when the smaller pore sized filter is used. This effect results from the fact that the bath fluid makes more passes through the filter and, presumably, the large particles trapped in the filter provide some filtering action also. However, this method still fails to remove a sufficient amount of the small particles for many types of devices and this problem will be exacerbated with the further diminution of device geometries. It is therefore apparent that a solution to the problem of particle removal is required wherein small particles can be removed in adequate amounts while filtration proceeds at a rapid rate.